At present, Same Frequency Networks (SNF) are used to broadcast data to users using a cellular structure.
In each cell, a base station may use the Orthogonal Frequency Division Multiplex (OFDM) modulation for efficient transmission of digital signals. These signals may include video, voice and/or data. OFDM is a commonly used implementation of Multi-Carrier Modulation (MCM). The Orthogonal Frequency Division Multiplex (OFDM) is a modern advanced modulation method, that achieves better use of the frequency spectrum. Digital Video Broadcasting (DVB), for example, uses the OFDM system.
The cellular structure may be used to achieve better coverage of a geographical area, using lower power transmitters.
In a Multiple Frequency Network (MFN), each cell transmits at a frequency that is different from that of adjacent cells. This greatly reduces interference between cells because of the frequency separation. An MFN implementation, however, is not efficient in frequency spectrum utilization, since a large bandwidth is required—the total bandwidth of the system is a multiple of the bandwidth allocated to each transmitter (that is, the bandwidth of each base station).
Same Frequency Networks (SNF) achieve a much better utilization of the frequency spectrum, by allowing all the base stations to operate at the same frequency.
The base stations all transmit the same information and use the same frequency range. This achieves good frequency utilization, since each base station may be allocated the full bandwidth available in the network.
This SNF system may achieve a lower level of interference: If a subscriber receives signals from just one base station, there is no interference problem.
A possible interference problem may occur where a subscriber concurrently receives signals from two base stations or more. Interference in this case may be reduced using multipath signal enhancement techniques.
The signals broadcast from all base stations are identical, since they use the same frequency band to transmit the same information. Whilst a subscriber receiver is synchronized to one base station, signals from another base station appear as the same signal delayed in time, like multipath. The undesired multipath signal may be canceled or attenuated using channel equalizer means, like a transversal filter. A channel equalizer may be used to reduce interference from one or more base stations.
A possible problem in the above network is that it only works in one direction, for disseminating identical data to all subscribers. It may be desirable to implement an interactive network, in which users transmit back information to the base station, and a base station may transmit personalized information to each user.
Such an interactive network may found many uses. It may allow users to connect to the Internet, for example, or to ask for various personalized services, in addition to the common broadcast signals.
In an interactive network, the signals transmitted from the various base stations are different from each other. The above method for interference reduction is no more effective.
A channel equalizer cannot be used to reduce interference from base stations nearby, where the signals received from several base stations are different from each other.
A subscriber could use two receivers, one for the OFDM broadcast and a second receiver for personalized data, however that may be a costly solution having various disadvantages. It may also require separate transmitters in the base station.
Prior art patents apparently do not address the above problems in SFN. Thus, Dent, U.S. Pat. No. 5,555,257, details a cellular/satellite communications system with improved frequency re-use. It aims to minimize co-channel interference.
The system comprises a plurality of mobile stations in communication with a ground based station via an orbiting satellite. The orbiting satellite transponds a combination of signals received from the mobile stations to the ground station. At the ground station, the combined signals are processing using matrix models to separate the plurality of mobile station signals. The coefficients of the matrix are chosen to minimize interference between channels. Communication in the reciprocal direction proceeds in like manner. The system may include directional antennas.
Bronte et al.,U.S. Pat. No. 5,621,720, discloses a multichannel alignment system and method. A DS3 level access, monitor and test system for a telephone network. The system provides selective, and hitless, bit overwrite in any of the embedded DS1, DS0 and subrate channels in a DS3 signal. Multiple DS0 and subrate channels can be tested via the asynchronous time slot interchange in conjunction with the recombiner of the present invention. A second embodiment of the invention partitions the system into a base subsystem, a communications link and a remote subsystem.
The distributed architecture system provides all of the performance monitoring and testing capabilities of the existing access system. The distributed architecture system provides a mechanism to transport a plurality of asynchronous and rate independent signals across the link to permit remote testing of digital and voice DS0 frequency circuits. In the preferred embodiment, the link that connects the base to the remote system is a standard DS1 channel. The clocks for the transmit and receive paths are derived from the same source, thus providing a common reference clock to both ends of the link.
Campana, Jr., U.S. Pat. No. 5,710,798, discloses a system for wireless transmission and receiving of information which is subject to fading by using a RF carrier modulated with a subcarrier modulated with the information. The system has a bus interface which communicates with a digital signal processor which controls the transmitting and receiving circuitry functions.
The digital signal processor provides first and second encoded information streams each comprising the information to be transmitted with the second stream being delayed by a time delay interval with respect to the first stream which is equal to or greater than the fading interval.
The first and second encoded information streams modulate cycles of the subcarrier to produce first and second parallel information streams which are time offset by the time delay interval.
The receiving circuitry has a detector for detecting transmitted first and second parallel information streams with the second parallel information stream being delayed from the first parallel information stream by the time delay interval. The digital signal processor determines if faded information is present in the frames of the detected first and second parallel information streams by processing the error correction code therein to determine if a number of bit errors are present which exceed the bit error correction capacity of the error correction code. The digital signal processor places an error marker within the detected first and second parallel information streams to mark each faded information unit and controls replacement of each error marker within at least one of the first and second parallel information streams with replacement data bits.
Dent, U.S. Pat. No. 5,631,898, discloses a cellular/satellite communications system with improved frequency re-use.
The system employs matrix processing of received and transmitted signals to minimize co-channel interference. It transmits signals to a plurality of remote units, using an updateable inverse C-matrix.
Panech et al., U.S. Pat. No. 5,657,358, discloses a subscriber RF telephone system for providing multiple speech and/or data signals simultaneously over either a single or plurality of RF channels.
It has a base station in communication with telephone lines and with mobile subscriber stations. The system comprises a switching matrix at the base station and set-up means at the subscriber stations. Means are provided for a periodic exchange of information during the course of communication operation concerning the present status for the connection between the base station and a subscriber station, the link quality, power, and timing adjustment thereof and providing adjustment to the subscriber station based thereupon.
Dent, U.S. Pat. No. 5,619,503, discloses a cellular/satellite communications system with improved frequency re-use, to minimize co-channel interference. A matrix processor is used to separate signals received from, or transmitted to, a plurality of mobile units. The coefficients of the matrix are chosen to minimize interference between channels, and are adjusted periodically.
The system rotates the allocation of Digital Voice Color Codes and sector radiation patterns in opposite directions to maintain the same color code in the same absolute direction.
Dent et al., U.S. Pat. No. 5,581,597, discloses a radio personal communications system and method for selecting a frequency for use in communications between a base station and a cellular terminal. Transceiver frequency and optionally power level are allocated to a radio personal communications system which includes a base station connected to a wire telephone network and a cellular terminal operating within a region of a wide area cellular network to minimize interference. The frequency for communications between the cellular terminal and the base station are optionally assigned by the operator of the wide area cellular network so appropriate frequencies and power levels can be assigned for base stations, to minimize same channel interference with the wide area cellular network.
Olds et al., U.S. Pat. No. 5,930,679, discloses a satellite-based ring alert apparatus and method of use. A fixed channel is defined wherein subscriber units are notified of incoming calls. Subscriber units monitor this ring alert channel and examine subscriber unit IDs transmitted in a ring alert message contained therein. When a subscriber unit determines from the subscriber unit ID that a call is being directed to it, it calls into a network to receive a call. Subscriber units also monitor this fixed channel to receive a control channel directory identifying the location of a subscriber unit's broadcast (control) channels.
Dent et al., U.S. Pat. No. 5,812,955, discloses a base station which relays cellular verification signals via a telephone wire network to verify a cellular radio telephone.
A secure radio personal communication system and method includes a base station which relays cellular verification signals between a wide area cellular network and a cellular terminal via the wire telephone network. Thus, cellular telephone calls which are routed to a cellular terminal via a base station, when the cellular terminal is within a local region covered by the base station, may be exchanged between the cellular network and cellular terminal over the wire telephone network.
Calls from the wide area cellular network which are routed through the base station can thus employ the same security systems and methods which are employed by the wide area cellular network.
Reis et al., U.S. Pat. No. 5,973,613, discloses a personal messaging system and method.
A pager device received paging messages. The user reads a paging message, selects one of a set of stored, predetermine reply messages, and transmits the selected reply message from the pager. The reply signal is received by one of a set of local cellular receivers, which sends the received signal to a computer for interpretation.
The pagers are associated with items located in a communication region that is interrogated by an interrogator on a one-to-many basis using broadcast commands, on a one-to-one basis using directed commands or on a combination basis using both methods.
Baran, U.S. Pat. No. 5,926,479, discloses a multiple protocol personal communications network system. A bi-directional communications system for bypassing a local exchange carrier telephone system for conveying data between at least one terminal unit and a switching unit. The system includes a relay transceiver where the signals from the terminal units are converted bi-directionally between the signal format of the terminal units and fixed length compliant ATM cells.
There can be at least two terminal units each designed to transmit and receive in different signal formats from each other and a programmable relay transceiver unit.
Weaver, Jr. et al., U.S. Pat. No. 5,828,661, discloses a method and apparatus for providing a cone of silence in a cellular communication system.
A remote unit communicates with another user via at least one base station. The network has a plurality of base stations controlled by a mobile switching center. Each base station of the plurality of base stations transmits an identifying pilot signal. To defining an area in which communication between a set of the base stations and the remote unit is forbidden, an auxiliary antenna transmits a silent region identifying pilot signal. The remote unit measures a signal strength of a set of identifying pilot signals corresponding to a neighbor set of base stations and measures a signal strength of the silent region identifying pilot signal. The remote unit sends a pilot strength measurement report to the mobile switching center via a first base station with which the remote unit has established communication.
Wang, U.S. Pat. No. 5,898,904, discloses a two-way wireless data network having a transmitter having a range greater than portions of the service areas. A two-way data network includes a broadcast control sub-network and a cellular data sub-network. The broadcast control sub-network includes a few high-power radio transmitters broadcasting into a large service area. The cellular data sub-network covers the large service area by a number of base stations each servicing a relatively small area. The broadcast control sub-network is used to notify the subscriber the receipt of a message.
In responding to the notification, the location of the subscriber's wireless terminal is make known, thereby eliminating conventional tasks such as location and mobility management tasks and allowing wireless terminals to be low power.